Due to gradual increase in demands on digital contents, an enormous amount of multimedia contents such as videos, music, and images are being continuously produced, manufactured, distributed, and serviced. Among them, use of digital still images (hereinafter, referred to as ‘still images’) is explosively increasing due to supply of portable digital cameras, mass storage units, and portable storage mediums, and their low prices. Accordingly, demands on products for searching for the still image if necessary and discriminating it from another one are gradually increasing. This technology is referred to as a still image search technology.
Until now, technologies for searching still images have been mainly based on meta data (keywords) or contents of the still images. The technologies utilize pre-input information or natural information contained in an original still image, such as a color and a texture of the still image, and are very useful in searching for the undeformed original still image.
However, in the above-mentioned conventional technologies for searching for a still image, when the still image is illegally deformed due to a copying thereof or an intended attack thereon, or when a natural characteristic of the still image is deformed by the Internet or types of reproduction devices, natural information itself contained in the still image may not be utilized.